/*
  Copyright (C) 2010 Perens LLC <bruce@perens.com>

  This program is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program.  If not, see <http://www.gnu.org/licenses/>.

 */
#include "defines.h"
#include "quantise.h"
#include <stdio.h>

/* Compile-time constants */
/* Size of unsigned char in bits. Assumes 8 bits-per-char. */
static const unsigned int	WordSize = 8;

/* Mask to pick the bit component out of bitIndex. */
static const unsigned int	IndexMask = 0x7;

/* Used to pick the word component out of bitIndex. */
static const unsigned int	ShiftRight = 3;

/** Pack a bit field into a bit string, encoding the field in Gray code.
 *
 * The output is an array of unsigned char data. The fields are efficiently
 * packed into the bit string. The Gray coding is a naive attempt to reduce
 * the effect of single-bit errors, we expect to do a better job as the
 * codec develops.
 *
 * This code would be simpler if it just set one bit at a time in the string,
 * but would hit the same cache line more often. I'm not sure the complexity
 * gains us anything here.
 *
 * Although field is currently of int type rather than unsigned for
 * compatibility with the rest of the code, indices are always expected to
 * be >= 0.
 */
void
pack(
 unsigned char *	bitArray, /* The output bit string. */
 unsigned int *		bitIndex, /* Index into the string in BITS, not bytes.*/
 int			field,	  /* The bit field to be packed. */
 unsigned int		fieldWidth/* Width of the field in BITS, not bytes. */
 )
{
  /* Convert the field to Gray code */
  field = (field >> 1) ^ field;

  do {
    unsigned int  	bI = *bitIndex;
    unsigned int	bitsLeft = WordSize - (bI & IndexMask);
    unsigned int	sliceWidth =
			 bitsLeft < fieldWidth ? bitsLeft : fieldWidth;
    unsigned int	wordIndex = bI >> ShiftRight;

    bitArray[wordIndex] |=
     ((unsigned char)((field >> (fieldWidth - sliceWidth))
     << (bitsLeft - sliceWidth)));
    
    *bitIndex = bI + sliceWidth;
    fieldWidth -= sliceWidth;
  } while ( fieldWidth != 0 );
}

/** Unpack a field from a bit string, converting from Gray code to binary.
 *
 */
int
unpack(
 const unsigned char *	bitArray, /* The input bit string. */
 unsigned int *		bitIndex, /* Index into the string in BITS, not bytes.*/
 unsigned int		fieldWidth/* Width of the field in BITS, not bytes. */
 )
{
  unsigned int	field = 0;
  unsigned int	t;

  do {
    unsigned int  	bI = *bitIndex;
    unsigned int	bitsLeft = WordSize - (bI & IndexMask);
    unsigned int	sliceWidth =
			 bitsLeft < fieldWidth ? bitsLeft : fieldWidth;

    field |= (((bitArray[bI >> ShiftRight] >> (bitsLeft - sliceWidth)) & ((1 << sliceWidth) - 1)) << (fieldWidth - sliceWidth));
    
    *bitIndex = bI + sliceWidth;
    fieldWidth -= sliceWidth;
  } while ( fieldWidth != 0 );

  /* Convert from Gray code to binary. Works for maximum 8-bit fields. */
  t = field ^ (field >> 8);
  t ^= (t >> 4);
  t ^= (t >> 2);
  t ^= (t >> 1);
  return t;
}
